The present invention relates to an oscillation circuit.
A conventional oscillation circuit may include an inverter formed of a MOS (Metal Oxide Semiconductor) transistor.
Patent Reference has disclosed an example of the conventional oscillation circuit. According to Patent Reference, the conventional oscillation circuit includes the inverter formed of a first P-channel MOS transistor and a first N-channel MOS transistor. Further, the conventional oscillation circuit includes a crystal oscillator connected between an input terminal and an output terminal of the inverter. Further, a second P-channel MOS transistor and a second N-channel MOS transistor are connected in series to a power source side and a ground side of the inverter, respectively.
Patent Reference: Japanese Patent Publication No. 06-45830
According to Patent Reference, the conventional oscillation circuit further includes a voltage adjusting circuit for adjusting a power source voltage. The voltage adjusting circuit is formed of a third MOS transistor and a resistor. The voltage adjusting circuit is connected to gates of the second P-channel MOS transistor and the second N-channel MOS transistor on the power source side and the ground side of the inverter, respectively. With the configuration as described above, the conventional oscillation circuit is capable of widening a selection range of a frequency that can be used relative to a voltage range of the conventional oscillation circuit.
The conventional oscillation circuit disclosed in Patent Reference may be combined with a functional circuit to constitute a system, so that the functional circuit operates according to an output signal of the conventional oscillation circuit as a clock signal. In this case, when the power source voltage to be supplied to the functional circuit is decreased, an operational speed of the functional circuit may be decreased. If this happens, the functional circuit is not able to operate in synchronization with the clock signal, thereby causing an operational problem.
In order to prevent the functional circuit from causing the operational problem when the power source voltage is decreased, it may be configured such that a frequency of the output signal of the conventional oscillation circuit is divided when the power source voltage is decreased. However, in this case, if the start timing of the frequency division is delayed, it is difficult to prevent the functional circuit from causing the operational problem.
In view of the problems of the conventional oscillation circuit described above, an object of the present invention is to provide an oscillation circuit capable of preventing a functional circuit from causing the operational problem when the oscillation circuit is provided for supplying a clock signal to the functional circuit, even if the functional circuit tends to delay in following the clock signal when the power source voltage is decreased.
Further objects and advantages of the invention will be apparent from the following description of the invention.